Without spoiling myself: What do the secret vault symbols that you generally have to find in areas for their Badass ranks look like? I've looked around for a while but couldn't find anything. It would be less of a waste of time if I knew what I was looking for.

This answer deals with finding an easy example in-game, and as such contains spoilers:

Go to the very first area (Claptrap's 'Home') where the beast pulls Claptrap's eye out and you're asked to search the locker or something for a gun. It's the first gun you get in-game, in the area with tons of dead Claptraps.

On the right side of that area is a wall you can open. A bunch of dead claptraps will burst out when you open it. The borderlands symbol is there to "discover" on the back wall.